In recent years considerable effort has been expended on delivering and conserving water as it has become an increasingly valuable and expensive resource. It has become necessary to develop more efficient systems for delivering water to sustain plant life. Most above-ground irrigation systems have utilized sprinkler devices for surface watering which is extremely wasteful in that the water flows off the surface and is not uniformly absorbed, or rapidly evaporates, only limited portions reaching the plant roots. The roots are the water-utilization system for most plants and must be present for their enzyme-water dependent processes to sustain plant life. In addition, above-ground watering practices interfere with use of recreational areas such as playing fields and golf courses, for example. Most importantly, surface water cannot be specifically directed to the desired plants or crops being applicable to both desired and undesired plant life, such as weeds.
Subsurface irrigation systems have been developed in which water, both with and without nourishing fertilizer, is directly delivered to the subsurface depth of the crop roots being cultivated. Such systems have employed flexible pipe which is inert to the soil conditions, and is capable of withstanding hydrostatic pressure without collapsing. The pipe may be readily bent to follow crop-line contours and elevations. The pipe which emits water slowly over its entire surface and length is buried to the proper depth underground for the particular crop or plant life being irrigated so that water is supplied directly to the plant roots. The amount of water delivered can be precisely controlled at or near optimum levels for the particular crop or plant life. In many cases crop yields have been substantially increased. Irrigation using porous pipe requires that the water be delivered uniformly along its length. The soil soaking process is generally similar to that obtained by drip irrigation; however, the leak rate is continuous along the pipe length rather than at discrete individual points or outlets as in drip irrigation.
Porous irrigation pipe produced from reclaimed rubber mixed with polyethylene as a binder has been disclosed by Turner in U.S. Pat. Nos. 4,003,408, 4,110,420 and 4,168,799. The prescribed mixture is extruded to form the pipe, and the water present within the hot extrudate vaporizes, producing the small pores through which water is emitted under low pressure. Such pipe presents problems in use due to its highly variable and uncontrolled porosity. Some sections have exhibited few or no pores while other sections have very large uneven pores. Water delivery rates from this pipe have varied from 50 to 75 percent along relatively short lengths indicating the product is unsuitable for close plantings. Thus, the overall porosity of the pipe is not sufficiently controlled for uniform water delivery resulting in problems when designing a subsurface water irrigation system for a prescribed location. It appears that the wide variation in porosity of this pipe product is primarily due to failure to control the moisture content of the extruded mixture. In the manufacture of this product attempts were made to control excess water in the mixture by venting the extruder, but this procedure did not result in controlling the variable porosity of the pipe. In addition, the vented extruder produces a pipe product having a rough internal surface which results in a high pressure drop along a length of pipe due to a high coefficient of friction and high Reynolds numbers for fluid flow. Such condition increases uneven water emission along a given length of porous pipe.
Other attempts to improve porous irrigation pipe have been made by Mason in U.S. Pat. Nos. 4,517,316, 4,615,642, and 4,616,055. In his processes the raw materials are preprocessed into shaped pellet form to provide a pre-formed feed material to the extruder which is less hygroscopic than powdered feed materials. However, his process is more costly and involved requiring the pelletizing steps. In addition, his moisture content of the pellets is much higher than that of the present invention ranging from 0.5 to 3.0 precent by weight of water. Such water content does not improve the uniformity of porosity of the pipe but exacerbates the problems of non-uniform porosity and uneven water emission. Mason in U.S. Pat. No. 4,517,316 utilizes additional constituents such as slip agents and lubricants, and excessive amounts of water in his pelletized mixture, and extrudes the pellets from the pipe extruder at an excessively high temperature. His porous pipe shows no improvement in porosity variation over that of the prior art, primarily due to excessive moisture in his pellets. Mason in U.S. Pat. No. 4,615,642 attempts to improve leak rate variation by varying the extruder die temperature and by varying the pull-off rate of the pipe. He seeks to controllably increase the porosity from first inlet end to second outlet end of the pipe length so that the leak rate is constant throughout the pipe length as the pressure drops between the ends. However, such constant varying of process variables does not produce an improved porous pipe. Mason in U.S. Pat. No. 4,616,055 also pelletizes the raw materials and employs additives to increase the flow of binder resin around the rubber particles; however, it does not result in more uniform porosity of the pipe. He coats the surface particles on the inner wall of the pipe to no avail in attempting to solve the porosity problems. In addition, all of Mason's porous pipe possesses length limitations in actual use despite his variable process procedures. His control of the water content of the raw materials in pellet form is inadequate for the intended purpose.